New way to manage reference counting of mbuf external storage.

[FreeBSD/FreeBSD.git] / sys / dev / xen / netback / netback.c

/*-
 * Copyright (c) 2009-2011 Spectra Logic Corporation
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions, and the following disclaimer,
 *    without modification.
 * 2. Redistributions in binary form must reproduce at minimum a disclaimer
 *    substantially similar to the "NO WARRANTY" disclaimer below
 *    ("Disclaimer") and any redistribution must be conditioned upon
 *    including a substantially similar Disclaimer requirement for further
 *    binary redistribution.
 *
 * NO WARRANTY
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * HOLDERS OR CONTRIBUTORS BE LIABLE FOR SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGES.
 *
 * Authors: Justin T. Gibbs     (Spectra Logic Corporation)
 *          Alan Somers         (Spectra Logic Corporation)
 *          John Suykerbuyk     (Spectra Logic Corporation)
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

/**
 * \file netback.c
 *
 * \brief Device driver supporting the vending of network access
 *        from this FreeBSD domain to other domains.
 */
#include "opt_inet.h"
#include "opt_inet6.h"

#include "opt_sctp.h"

#include <sys/param.h>
#include <sys/kernel.h>

#include <sys/bus.h>
#include <sys/module.h>
#include <sys/rman.h>
#include <sys/socket.h>
#include <sys/sockio.h>
#include <sys/sysctl.h>

#include <net/if.h>
#include <net/if_var.h>
#include <net/if_arp.h>
#include <net/ethernet.h>
#include <net/if_dl.h>
#include <net/if_media.h>
#include <net/if_types.h>

#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/if_ether.h>
#if __FreeBSD_version >= 700000
#include <netinet/tcp.h>
#endif
#include <netinet/ip_icmp.h>
#include <netinet/udp.h>
#include <machine/in_cksum.h>

#include <vm/vm.h>
#include <vm/pmap.h>
#include <vm/vm_extern.h>
#include <vm/vm_kern.h>

#include <machine/_inttypes.h>

#include <xen/xen-os.h>
#include <xen/hypervisor.h>
#include <xen/xen_intr.h>
#include <xen/interface/io/netif.h>
#include <xen/xenbus/xenbusvar.h>

/*--------------------------- Compile-time Tunables --------------------------*/

/*---------------------------------- Macros ----------------------------------*/
/**
 * Custom malloc type for all driver allocations.
 */
static MALLOC_DEFINE(M_XENNETBACK, "xnb", "Xen Net Back Driver Data");

#define XNB_SG  1       /* netback driver supports feature-sg */
#define XNB_GSO_TCPV4 0 /* netback driver supports feature-gso-tcpv4 */
#define XNB_RX_COPY 1   /* netback driver supports feature-rx-copy */
#define XNB_RX_FLIP 0   /* netback driver does not support feature-rx-flip */

#undef XNB_DEBUG
#define XNB_DEBUG /* hardcode on during development */

#ifdef XNB_DEBUG
#define DPRINTF(fmt, args...) \
        printf("xnb(%s:%d): " fmt, __FUNCTION__, __LINE__, ##args)
#else
#define DPRINTF(fmt, args...) do {} while (0)
#endif

/* Default length for stack-allocated grant tables */
#define GNTTAB_LEN      (64)

/* Features supported by all backends.  TSO and LRO can be negotiated */
#define XNB_CSUM_FEATURES       (CSUM_TCP | CSUM_UDP)

#define NET_TX_RING_SIZE __RING_SIZE((netif_tx_sring_t *)0, PAGE_SIZE)
#define NET_RX_RING_SIZE __RING_SIZE((netif_rx_sring_t *)0, PAGE_SIZE)

/**
 * Two argument version of the standard macro.  Second argument is a tentative
 * value of req_cons
 */
#define RING_HAS_UNCONSUMED_REQUESTS_2(_r, cons) ({                     \
        unsigned int req = (_r)->sring->req_prod - cons;                \
        unsigned int rsp = RING_SIZE(_r) -                              \
        (cons - (_r)->rsp_prod_pvt);                                    \
        req < rsp ? req : rsp;                                          \
})

#define virt_to_mfn(x) (vtophys(x) >> PAGE_SHIFT)
#define virt_to_offset(x) ((x) & (PAGE_SIZE - 1))

/**
 * Predefined array type of grant table copy descriptors.  Used to pass around
 * statically allocated memory structures.
 */
typedef struct gnttab_copy gnttab_copy_table[GNTTAB_LEN];

/*--------------------------- Forward Declarations ---------------------------*/
struct xnb_softc;
struct xnb_pkt;

static void     xnb_attach_failed(struct xnb_softc *xnb,
                                  int err, const char *fmt, ...)
                                  __printflike(3,4);
static int      xnb_shutdown(struct xnb_softc *xnb);
static int      create_netdev(device_t dev);
static int      xnb_detach(device_t dev);
static int      xnb_ifmedia_upd(struct ifnet *ifp);
static void     xnb_ifmedia_sts(struct ifnet *ifp, struct ifmediareq *ifmr);
static void     xnb_intr(void *arg);
static int      xnb_send(netif_rx_back_ring_t *rxb, domid_t otherend,
                         const struct mbuf *mbufc, gnttab_copy_table gnttab);
static int      xnb_recv(netif_tx_back_ring_t *txb, domid_t otherend,
                         struct mbuf **mbufc, struct ifnet *ifnet,
                         gnttab_copy_table gnttab);
static int      xnb_ring2pkt(struct xnb_pkt *pkt,
                             const netif_tx_back_ring_t *tx_ring,
                             RING_IDX start);
static void     xnb_txpkt2rsp(const struct xnb_pkt *pkt,
                              netif_tx_back_ring_t *ring, int error);
static struct mbuf *xnb_pkt2mbufc(const struct xnb_pkt *pkt, struct ifnet *ifp);
static int      xnb_txpkt2gnttab(const struct xnb_pkt *pkt,
                                 struct mbuf *mbufc,
                                 gnttab_copy_table gnttab,
                                 const netif_tx_back_ring_t *txb,
                                 domid_t otherend_id);
static void     xnb_update_mbufc(struct mbuf *mbufc,
                                 const gnttab_copy_table gnttab, int n_entries);
static int      xnb_mbufc2pkt(const struct mbuf *mbufc,
                              struct xnb_pkt *pkt,
                              RING_IDX start, int space);
static int      xnb_rxpkt2gnttab(const struct xnb_pkt *pkt,
                                 const struct mbuf *mbufc,
                                 gnttab_copy_table gnttab,
                                 const netif_rx_back_ring_t *rxb,
                                 domid_t otherend_id);
static int      xnb_rxpkt2rsp(const struct xnb_pkt *pkt,
                              const gnttab_copy_table gnttab, int n_entries,
                              netif_rx_back_ring_t *ring);
static void     xnb_stop(struct xnb_softc*);
static int      xnb_ioctl(struct ifnet*, u_long, caddr_t);
static void     xnb_start_locked(struct ifnet*);
static void     xnb_start(struct ifnet*);
static void     xnb_ifinit_locked(struct xnb_softc*);
static void     xnb_ifinit(void*);
#ifdef XNB_DEBUG
static int      xnb_unit_test_main(SYSCTL_HANDLER_ARGS);
static int      xnb_dump_rings(SYSCTL_HANDLER_ARGS);
#endif
#if defined(INET) || defined(INET6)
static void     xnb_add_mbuf_cksum(struct mbuf *mbufc);
#endif
/*------------------------------ Data Structures -----------------------------*/


/**
 * Representation of a xennet packet.  Simplified version of a packet as
 * stored in the Xen tx ring.  Applicable to both RX and TX packets
 */
struct xnb_pkt{
        /**
         * Array index of the first data-bearing (eg, not extra info) entry
         * for this packet
         */
        RING_IDX        car;

        /**
         * Array index of the second data-bearing entry for this packet.
         * Invalid if the packet has only one data-bearing entry.  If the
         * packet has more than two data-bearing entries, then the second
         * through the last will be sequential modulo the ring size
         */
        RING_IDX        cdr;

        /**
         * Optional extra info.  Only valid if flags contains
         * NETTXF_extra_info.  Note that extra.type will always be
         * XEN_NETIF_EXTRA_TYPE_GSO.  Currently, no known netfront or netback
         * driver will ever set XEN_NETIF_EXTRA_TYPE_MCAST_*
         */
        netif_extra_info_t extra;

        /** Size of entire packet in bytes.       */
        uint16_t        size;

        /** The size of the first entry's data in bytes */
        uint16_t        car_size;

        /**
         * Either NETTXF_ or NETRXF_ flags.  Note that the flag values are
         * not the same for TX and RX packets
         */
        uint16_t        flags;

        /**
         * The number of valid data-bearing entries (either netif_tx_request's
         * or netif_rx_response's) in the packet.  If this is 0, it means the
         * entire packet is invalid.
         */
        uint16_t        list_len;

        /** There was an error processing the packet */
        uint8_t         error;
};

/** xnb_pkt method: initialize it */
static inline void
xnb_pkt_initialize(struct xnb_pkt *pxnb)
{
        bzero(pxnb, sizeof(*pxnb));
}

/** xnb_pkt method: mark the packet as valid */
static inline void
xnb_pkt_validate(struct xnb_pkt *pxnb)
{
        pxnb->error = 0;
};

/** xnb_pkt method: mark the packet as invalid */
static inline void
xnb_pkt_invalidate(struct xnb_pkt *pxnb)
{
        pxnb->error = 1;
};

/** xnb_pkt method: Check whether the packet is valid */
static inline int
xnb_pkt_is_valid(const struct xnb_pkt *pxnb)
{
        return (! pxnb->error);
}

#ifdef XNB_DEBUG
/** xnb_pkt method: print the packet's contents in human-readable format*/
static void __unused
xnb_dump_pkt(const struct xnb_pkt *pkt) {
        if (pkt == NULL) {
          DPRINTF("Was passed a null pointer.\n");
          return;
        }
        DPRINTF("pkt address= %p\n", pkt);
        DPRINTF("pkt->size=%d\n", pkt->size);
        DPRINTF("pkt->car_size=%d\n", pkt->car_size);
        DPRINTF("pkt->flags=0x%04x\n", pkt->flags);
        DPRINTF("pkt->list_len=%d\n", pkt->list_len);
        /* DPRINTF("pkt->extra");       TODO */
        DPRINTF("pkt->car=%d\n", pkt->car);
        DPRINTF("pkt->cdr=%d\n", pkt->cdr);
        DPRINTF("pkt->error=%d\n", pkt->error);
}
#endif /* XNB_DEBUG */

static void
xnb_dump_txreq(RING_IDX idx, const struct netif_tx_request *txreq)
{
        if (txreq != NULL) {
                DPRINTF("netif_tx_request index =%u\n", idx);
                DPRINTF("netif_tx_request.gref  =%u\n", txreq->gref);
                DPRINTF("netif_tx_request.offset=%hu\n", txreq->offset);
                DPRINTF("netif_tx_request.flags =%hu\n", txreq->flags);
                DPRINTF("netif_tx_request.id    =%hu\n", txreq->id);
                DPRINTF("netif_tx_request.size  =%hu\n", txreq->size);
        }
}


/**
 * \brief Configuration data for a shared memory request ring
 *        used to communicate with the front-end client of this
 *        this driver.
 */
struct xnb_ring_config {
        /**
         * Runtime structures for ring access.  Unfortunately, TX and RX rings
         * use different data structures, and that cannot be changed since it
         * is part of the interdomain protocol.
         */
        union{
                netif_rx_back_ring_t      rx_ring;
                netif_tx_back_ring_t      tx_ring;
        } back_ring;

        /**
         * The device bus address returned by the hypervisor when
         * mapping the ring and required to unmap it when a connection
         * is torn down.
         */
        uint64_t        bus_addr;

        /** The pseudo-physical address where ring memory is mapped.*/
        uint64_t        gnt_addr;

        /** KVA address where ring memory is mapped. */
        vm_offset_t     va;

        /**
         * Grant table handles, one per-ring page, returned by the
         * hyperpervisor upon mapping of the ring and required to
         * unmap it when a connection is torn down.
         */
        grant_handle_t  handle;

        /** The number of ring pages mapped for the current connection. */
        unsigned        ring_pages;

        /**
         * The grant references, one per-ring page, supplied by the
         * front-end, allowing us to reference the ring pages in the
         * front-end's domain and to map these pages into our own domain.
         */
        grant_ref_t     ring_ref;
};

/**
 * Per-instance connection state flags.
 */
typedef enum
{
        /** Communication with the front-end has been established. */
        XNBF_RING_CONNECTED    = 0x01,

        /**
         * Front-end requests exist in the ring and are waiting for
         * xnb_xen_req objects to free up.
         */
        XNBF_RESOURCE_SHORTAGE = 0x02,

        /** Connection teardown has started. */
        XNBF_SHUTDOWN          = 0x04,

        /** A thread is already performing shutdown processing. */
        XNBF_IN_SHUTDOWN       = 0x08
} xnb_flag_t;

/**
 * Types of rings.  Used for array indices and to identify a ring's control
 * data structure type
 */
typedef enum{
        XNB_RING_TYPE_TX = 0,   /* ID of TX rings, used for array indices */
        XNB_RING_TYPE_RX = 1,   /* ID of RX rings, used for array indices */
        XNB_NUM_RING_TYPES
} xnb_ring_type_t;

/**
 * Per-instance configuration data.
 */
struct xnb_softc {
        /** NewBus device corresponding to this instance. */
        device_t                dev;

        /* Media related fields */

        /** Generic network media state */
        struct ifmedia          sc_media;

        /** Media carrier info */
        struct ifnet            *xnb_ifp;

        /** Our own private carrier state */
        unsigned carrier;

        /** Device MAC Address */
        uint8_t                 mac[ETHER_ADDR_LEN];

        /* Xen related fields */

        /**
         * \brief The netif protocol abi in effect.
         *
         * There are situations where the back and front ends can
         * have a different, native abi (e.g. intel x86_64 and
         * 32bit x86 domains on the same machine).  The back-end
         * always accomodates the front-end's native abi.  That
         * value is pulled from the XenStore and recorded here.
         */
        int                     abi;

        /**
         * Name of the bridge to which this VIF is connected, if any
         * This field is dynamically allocated by xenbus and must be free()ed
         * when no longer needed
         */
        char                    *bridge;

        /** The interrupt driven even channel used to signal ring events. */
        evtchn_port_t           evtchn;

        /** Xen device handle.*/
        long                    handle;

        /** Handle to the communication ring event channel. */
        xen_intr_handle_t       xen_intr_handle;

        /**
         * \brief Cached value of the front-end's domain id.
         *
         * This value is used at once for each mapped page in
         * a transaction.  We cache it to avoid incuring the
         * cost of an ivar access every time this is needed.
         */
        domid_t                 otherend_id;

        /**
         * Undocumented frontend feature.  Has something to do with
         * scatter/gather IO
         */
        uint8_t                 can_sg;
        /** Undocumented frontend feature */
        uint8_t                 gso;
        /** Undocumented frontend feature */
        uint8_t                 gso_prefix;
        /** Can checksum TCP/UDP over IPv4 */
        uint8_t                 ip_csum;

        /* Implementation related fields */
        /**
         * Preallocated grant table copy descriptor for RX operations.
         * Access must be protected by rx_lock
         */
        gnttab_copy_table       rx_gnttab;

        /**
         * Preallocated grant table copy descriptor for TX operations.
         * Access must be protected by tx_lock
         */
        gnttab_copy_table       tx_gnttab;

        /**
         * Resource representing allocated physical address space
         * associated with our per-instance kva region.
         */
        struct resource         *pseudo_phys_res;

        /** Resource id for allocated physical address space. */
        int                     pseudo_phys_res_id;

        /** Ring mapping and interrupt configuration data. */
        struct xnb_ring_config  ring_configs[XNB_NUM_RING_TYPES];

        /**
         * Global pool of kva used for mapping remote domain ring
         * and I/O transaction data.
         */
        vm_offset_t             kva;

        /** Psuedo-physical address corresponding to kva. */
        uint64_t                gnt_base_addr;

        /** Various configuration and state bit flags. */
        xnb_flag_t              flags;

        /** Mutex protecting per-instance data in the receive path. */
        struct mtx              rx_lock;

        /** Mutex protecting per-instance data in the softc structure. */
        struct mtx              sc_lock;

        /** Mutex protecting per-instance data in the transmit path. */
        struct mtx              tx_lock;

        /** The size of the global kva pool. */
        int                     kva_size;

        /** Name of the interface */
        char                     if_name[IFNAMSIZ];
};

/*---------------------------- Debugging functions ---------------------------*/
#ifdef XNB_DEBUG
static void __unused
xnb_dump_gnttab_copy(const struct gnttab_copy *entry)
{
        if (entry == NULL) {
                printf("NULL grant table pointer\n");
                return;
        }

        if (entry->flags & GNTCOPY_dest_gref)
                printf("gnttab dest ref=\t%u\n", entry->dest.u.ref);
        else
                printf("gnttab dest gmfn=\t%"PRI_xen_pfn"\n",
                       entry->dest.u.gmfn);
        printf("gnttab dest offset=\t%hu\n", entry->dest.offset);
        printf("gnttab dest domid=\t%hu\n", entry->dest.domid);
        if (entry->flags & GNTCOPY_source_gref)
                printf("gnttab source ref=\t%u\n", entry->source.u.ref);
        else
                printf("gnttab source gmfn=\t%"PRI_xen_pfn"\n",
                       entry->source.u.gmfn);
        printf("gnttab source offset=\t%hu\n", entry->source.offset);
        printf("gnttab source domid=\t%hu\n", entry->source.domid);
        printf("gnttab len=\t%hu\n", entry->len);
        printf("gnttab flags=\t%hu\n", entry->flags);
        printf("gnttab status=\t%hd\n", entry->status);
}

static int
xnb_dump_rings(SYSCTL_HANDLER_ARGS)
{
        static char results[720];
        struct xnb_softc const* xnb = (struct xnb_softc*)arg1;
        netif_rx_back_ring_t const* rxb =
                &xnb->ring_configs[XNB_RING_TYPE_RX].back_ring.rx_ring;
        netif_tx_back_ring_t const* txb =
                &xnb->ring_configs[XNB_RING_TYPE_TX].back_ring.tx_ring;

        /* empty the result strings */
        results[0] = 0;

        if ( !txb || !txb->sring || !rxb || !rxb->sring )
                return (SYSCTL_OUT(req, results, strnlen(results, 720)));

        snprintf(results, 720,
            "\n\t%35s %18s\n"   /* TX, RX */
            "\t%16s %18d %18d\n"        /* req_cons */
            "\t%16s %18d %18d\n"        /* nr_ents */
            "\t%16s %18d %18d\n"        /* rsp_prod_pvt */
            "\t%16s %18p %18p\n"        /* sring */
            "\t%16s %18d %18d\n"        /* req_prod */
            "\t%16s %18d %18d\n"        /* req_event */
            "\t%16s %18d %18d\n"        /* rsp_prod */
            "\t%16s %18d %18d\n",       /* rsp_event */
            "TX", "RX",
            "req_cons", txb->req_cons, rxb->req_cons,
            "nr_ents", txb->nr_ents, rxb->nr_ents,
            "rsp_prod_pvt", txb->rsp_prod_pvt, rxb->rsp_prod_pvt,
            "sring", txb->sring, rxb->sring,
            "sring->req_prod", txb->sring->req_prod, rxb->sring->req_prod,
            "sring->req_event", txb->sring->req_event, rxb->sring->req_event,
            "sring->rsp_prod", txb->sring->rsp_prod, rxb->sring->rsp_prod,
            "sring->rsp_event", txb->sring->rsp_event, rxb->sring->rsp_event);

        return (SYSCTL_OUT(req, results, strnlen(results, 720)));
}

static void __unused
xnb_dump_mbuf(const struct mbuf *m)
{
        int len;
        uint8_t *d;
        if (m == NULL)
                return;

        printf("xnb_dump_mbuf:\n");
        if (m->m_flags & M_PKTHDR) {
                printf("    flowid=%10d, csum_flags=%#8x, csum_data=%#8x, "
                       "tso_segsz=%5hd\n",
                       m->m_pkthdr.flowid, (int)m->m_pkthdr.csum_flags,
                       m->m_pkthdr.csum_data, m->m_pkthdr.tso_segsz);
                printf("    rcvif=%16p,  len=%19d\n",
                       m->m_pkthdr.rcvif, m->m_pkthdr.len);
        }
        printf("    m_next=%16p, m_nextpk=%16p, m_data=%16p\n",
               m->m_next, m->m_nextpkt, m->m_data);
        printf("    m_len=%17d, m_flags=%#15x, m_type=%18u\n",
               m->m_len, m->m_flags, m->m_type);

        len = m->m_len;
        d = mtod(m, uint8_t*);
        while (len > 0) {
                int i;
                printf("                ");
                for (i = 0; (i < 16) && (len > 0); i++, len--) {
                        printf("%02hhx ", *(d++));
                }
                printf("\n");
        }
}
#endif /* XNB_DEBUG */

/*------------------------ Inter-Domain Communication ------------------------*/
/**
 * Free dynamically allocated KVA or pseudo-physical address allocations.
 *
 * \param xnb  Per-instance xnb configuration structure.
 */
static void
xnb_free_communication_mem(struct xnb_softc *xnb)
{
        if (xnb->kva != 0) {
                if (xnb->pseudo_phys_res != NULL) {
                        xenmem_free(xnb->dev, xnb->pseudo_phys_res_id,
                            xnb->pseudo_phys_res);
                        xnb->pseudo_phys_res = NULL;
                }
        }
        xnb->kva = 0;
        xnb->gnt_base_addr = 0;
}

/**
 * Cleanup all inter-domain communication mechanisms.
 *
 * \param xnb  Per-instance xnb configuration structure.
 */
static int
xnb_disconnect(struct xnb_softc *xnb)
{
        struct gnttab_unmap_grant_ref gnts[XNB_NUM_RING_TYPES];
        int error;
        int i;

        if (xnb->xen_intr_handle != NULL)
                xen_intr_unbind(&xnb->xen_intr_handle);

        /*
         * We may still have another thread currently processing requests.  We
         * must acquire the rx and tx locks to make sure those threads are done,
         * but we can release those locks as soon as we acquire them, because no
         * more interrupts will be arriving.
         */
        mtx_lock(&xnb->tx_lock);
        mtx_unlock(&xnb->tx_lock);
        mtx_lock(&xnb->rx_lock);
        mtx_unlock(&xnb->rx_lock);

        /* Free malloc'd softc member variables */
        if (xnb->bridge != NULL) {
                free(xnb->bridge, M_XENSTORE);
                xnb->bridge = NULL;
        }

        /* All request processing has stopped, so unmap the rings */
        for (i=0; i < XNB_NUM_RING_TYPES; i++) {
                gnts[i].host_addr = xnb->ring_configs[i].gnt_addr;
                gnts[i].dev_bus_addr = xnb->ring_configs[i].bus_addr;
                gnts[i].handle = xnb->ring_configs[i].handle;
        }
        error = HYPERVISOR_grant_table_op(GNTTABOP_unmap_grant_ref, gnts,
                                          XNB_NUM_RING_TYPES);
        KASSERT(error == 0, ("Grant table unmap op failed (%d)", error));

        xnb_free_communication_mem(xnb);
        /*
         * Zero the ring config structs because the pointers, handles, and
         * grant refs contained therein are no longer valid.
         */
        bzero(&xnb->ring_configs[XNB_RING_TYPE_TX],
            sizeof(struct xnb_ring_config));
        bzero(&xnb->ring_configs[XNB_RING_TYPE_RX],
            sizeof(struct xnb_ring_config));

        xnb->flags &= ~XNBF_RING_CONNECTED;
        return (0);
}

/**
 * Map a single shared memory ring into domain local address space and
 * initialize its control structure
 *
 * \param xnb   Per-instance xnb configuration structure
 * \param ring_type     Array index of this ring in the xnb's array of rings
 * \return      An errno
 */
static int
xnb_connect_ring(struct xnb_softc *xnb, xnb_ring_type_t ring_type)
{
        struct gnttab_map_grant_ref gnt;
        struct xnb_ring_config *ring = &xnb->ring_configs[ring_type];
        int error;

        /* TX ring type = 0, RX =1 */
        ring->va = xnb->kva + ring_type * PAGE_SIZE;
        ring->gnt_addr = xnb->gnt_base_addr + ring_type * PAGE_SIZE;

        gnt.host_addr = ring->gnt_addr;
        gnt.flags     = GNTMAP_host_map;
        gnt.ref       = ring->ring_ref;
        gnt.dom       = xnb->otherend_id;

        error = HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, &gnt, 1);
        if (error != 0)
                panic("netback: Ring page grant table op failed (%d)", error);

        if (gnt.status != 0) {
                ring->va = 0;
                error = EACCES;
                xenbus_dev_fatal(xnb->dev, error,
                                 "Ring shared page mapping failed. "
                                 "Status %d.", gnt.status);
        } else {
                ring->handle = gnt.handle;
                ring->bus_addr = gnt.dev_bus_addr;

                if (ring_type == XNB_RING_TYPE_TX) {
                        BACK_RING_INIT(&ring->back_ring.tx_ring,
                            (netif_tx_sring_t*)ring->va,
                            ring->ring_pages * PAGE_SIZE);
                } else if (ring_type == XNB_RING_TYPE_RX) {
                        BACK_RING_INIT(&ring->back_ring.rx_ring,
                            (netif_rx_sring_t*)ring->va,
                            ring->ring_pages * PAGE_SIZE);
                } else {
                        xenbus_dev_fatal(xnb->dev, error,
                                 "Unknown ring type %d", ring_type);
                }
        }

        return error;
}

/**
 * Setup the shared memory rings and bind an interrupt to the event channel
 * used to notify us of ring changes.
 *
 * \param xnb  Per-instance xnb configuration structure.
 */
static int
xnb_connect_comms(struct xnb_softc *xnb)
{
        int     error;
        xnb_ring_type_t i;

        if ((xnb->flags & XNBF_RING_CONNECTED) != 0)
                return (0);

        /*
         * Kva for our rings are at the tail of the region of kva allocated
         * by xnb_alloc_communication_mem().
         */
        for (i=0; i < XNB_NUM_RING_TYPES; i++) {
                error = xnb_connect_ring(xnb, i);
                if (error != 0)
                        return error;
        }

        xnb->flags |= XNBF_RING_CONNECTED;

        error = xen_intr_bind_remote_port(xnb->dev,
                                          xnb->otherend_id,
                                          xnb->evtchn,
                                          /*filter*/NULL,
                                          xnb_intr, /*arg*/xnb,
                                          INTR_TYPE_BIO | INTR_MPSAFE,
                                          &xnb->xen_intr_handle);
        if (error != 0) {
                (void)xnb_disconnect(xnb);
                xenbus_dev_fatal(xnb->dev, error, "binding event channel");
                return (error);
        }

        DPRINTF("rings connected!\n");

        return (0);
}

/**
 * Size KVA and pseudo-physical address allocations based on negotiated
 * values for the size and number of I/O requests, and the size of our
 * communication ring.
 *
 * \param xnb  Per-instance xnb configuration structure.
 *
 * These address spaces are used to dynamically map pages in the
 * front-end's domain into our own.
 */
static int
xnb_alloc_communication_mem(struct xnb_softc *xnb)
{
        xnb_ring_type_t i;

        xnb->kva_size = 0;
        for (i=0; i < XNB_NUM_RING_TYPES; i++) {
                xnb->kva_size += xnb->ring_configs[i].ring_pages * PAGE_SIZE;
        }

        /*
         * Reserve a range of pseudo physical memory that we can map
         * into kva.  These pages will only be backed by machine
         * pages ("real memory") during the lifetime of front-end requests
         * via grant table operations.  We will map the netif tx and rx rings
         * into this space.
         */
        xnb->pseudo_phys_res_id = 0;
        xnb->pseudo_phys_res = xenmem_alloc(xnb->dev, &xnb->pseudo_phys_res_id,
            xnb->kva_size);
        if (xnb->pseudo_phys_res == NULL) {
                xnb->kva = 0;
                return (ENOMEM);
        }
        xnb->kva = (vm_offset_t)rman_get_virtual(xnb->pseudo_phys_res);
        xnb->gnt_base_addr = rman_get_start(xnb->pseudo_phys_res);
        return (0);
}

/**
 * Collect information from the XenStore related to our device and its frontend
 *
 * \param xnb  Per-instance xnb configuration structure.
 */
static int
xnb_collect_xenstore_info(struct xnb_softc *xnb)
{
        /**
         * \todo Linux collects the following info.  We should collect most
         * of this, too:
         * "feature-rx-notify"
         */
        const char *otherend_path;
        const char *our_path;
        int err;
        unsigned int rx_copy, bridge_len;
        uint8_t no_csum_offload;

        otherend_path = xenbus_get_otherend_path(xnb->dev);
        our_path = xenbus_get_node(xnb->dev);

        /* Collect the critical communication parameters */
        err = xs_gather(XST_NIL, otherend_path,
            "tx-ring-ref", "%l" PRIu32,
                &xnb->ring_configs[XNB_RING_TYPE_TX].ring_ref,
            "rx-ring-ref", "%l" PRIu32,
                &xnb->ring_configs[XNB_RING_TYPE_RX].ring_ref,
            "event-channel", "%" PRIu32, &xnb->evtchn,
            NULL);
        if (err != 0) {
                xenbus_dev_fatal(xnb->dev, err,
                                 "Unable to retrieve ring information from "
                                 "frontend %s.  Unable to connect.",
                                 otherend_path);
                return (err);
        }

        /* Collect the handle from xenstore */
        err = xs_scanf(XST_NIL, our_path, "handle", NULL, "%li", &xnb->handle);
        if (err != 0) {
                xenbus_dev_fatal(xnb->dev, err,
                    "Error reading handle from frontend %s.  "
                    "Unable to connect.", otherend_path);
        }

        /*
         * Collect the bridgename, if any.  We do not need bridge_len; we just
         * throw it away
         */
        err = xs_read(XST_NIL, our_path, "bridge", &bridge_len,
                      (void**)&xnb->bridge);
        if (err != 0)
                xnb->bridge = NULL;

        /*
         * Does the frontend request that we use rx copy?  If not, return an
         * error because this driver only supports rx copy.
         */
        err = xs_scanf(XST_NIL, otherend_path, "request-rx-copy", NULL,
                       "%" PRIu32, &rx_copy);
        if (err == ENOENT) {
                err = 0;
                rx_copy = 0;
        }
        if (err < 0) {
                xenbus_dev_fatal(xnb->dev, err, "reading %s/request-rx-copy",
                                 otherend_path);
                return err;
        }
        /**
         * \todo: figure out the exact meaning of this feature, and when
         * the frontend will set it to true.  It should be set to true
         * at some point
         */
/*        if (!rx_copy)*/
/*          return EOPNOTSUPP;*/

        /** \todo Collect the rx notify feature */

        /*  Collect the feature-sg. */
        if (xs_scanf(XST_NIL, otherend_path, "feature-sg", NULL,
                     "%hhu", &xnb->can_sg) < 0)
                xnb->can_sg = 0;

        /* Collect remaining frontend features */
        if (xs_scanf(XST_NIL, otherend_path, "feature-gso-tcpv4", NULL,
                     "%hhu", &xnb->gso) < 0)
                xnb->gso = 0;

        if (xs_scanf(XST_NIL, otherend_path, "feature-gso-tcpv4-prefix", NULL,
                     "%hhu", &xnb->gso_prefix) < 0)
                xnb->gso_prefix = 0;

        if (xs_scanf(XST_NIL, otherend_path, "feature-no-csum-offload", NULL,
                     "%hhu", &no_csum_offload) < 0)
                no_csum_offload = 0;
        xnb->ip_csum = (no_csum_offload == 0);

        return (0);
}

/**
 * Supply information about the physical device to the frontend
 * via XenBus.
 *
 * \param xnb  Per-instance xnb configuration structure.
 */
static int
xnb_publish_backend_info(struct xnb_softc *xnb)
{
        struct xs_transaction xst;
        const char *our_path;
        int error;

        our_path = xenbus_get_node(xnb->dev);

        do {
                error = xs_transaction_start(&xst);
                if (error != 0) {
                        xenbus_dev_fatal(xnb->dev, error,
                                         "Error publishing backend info "
                                         "(start transaction)");
                        break;
                }

                error = xs_printf(xst, our_path, "feature-sg",
                                  "%d", XNB_SG);
                if (error != 0)
                        break;

                error = xs_printf(xst, our_path, "feature-gso-tcpv4",
                                  "%d", XNB_GSO_TCPV4);
                if (error != 0)
                        break;

                error = xs_printf(xst, our_path, "feature-rx-copy",
                                  "%d", XNB_RX_COPY);
                if (error != 0)
                        break;

                error = xs_printf(xst, our_path, "feature-rx-flip",
                                  "%d", XNB_RX_FLIP);
                if (error != 0)
                        break;

                error = xs_transaction_end(xst, 0);
                if (error != 0 && error != EAGAIN) {
                        xenbus_dev_fatal(xnb->dev, error, "ending transaction");
                        break;
                }

        } while (error == EAGAIN);

        return (error);
}

/**
 * Connect to our netfront peer now that it has completed publishing
 * its configuration into the XenStore.
 *
 * \param xnb  Per-instance xnb configuration structure.
 */
static void
xnb_connect(struct xnb_softc *xnb)
{
        int     error;

        if (xenbus_get_state(xnb->dev) == XenbusStateConnected)
                return;

        if (xnb_collect_xenstore_info(xnb) != 0)
                return;

        xnb->flags &= ~XNBF_SHUTDOWN;

        /* Read front end configuration. */

        /* Allocate resources whose size depends on front-end configuration. */
        error = xnb_alloc_communication_mem(xnb);
        if (error != 0) {
                xenbus_dev_fatal(xnb->dev, error,
                                 "Unable to allocate communication memory");
                return;
        }

        /*
         * Connect communication channel.
         */
        error = xnb_connect_comms(xnb);
        if (error != 0) {
                /* Specific errors are reported by xnb_connect_comms(). */
                return;
        }
        xnb->carrier = 1;

        /* Ready for I/O. */
        xenbus_set_state(xnb->dev, XenbusStateConnected);
}

/*-------------------------- Device Teardown Support -------------------------*/
/**
 * Perform device shutdown functions.
 *
 * \param xnb  Per-instance xnb configuration structure.
 *
 * Mark this instance as shutting down, wait for any active requests
 * to drain, disconnect from the front-end, and notify any waiters (e.g.
 * a thread invoking our detach method) that detach can now proceed.
 */
static int
xnb_shutdown(struct xnb_softc *xnb)
{
        /*
         * Due to the need to drop our mutex during some
         * xenbus operations, it is possible for two threads
         * to attempt to close out shutdown processing at
         * the same time.  Tell the caller that hits this
         * race to try back later.
         */
        if ((xnb->flags & XNBF_IN_SHUTDOWN) != 0)
                return (EAGAIN);

        xnb->flags |= XNBF_SHUTDOWN;

        xnb->flags |= XNBF_IN_SHUTDOWN;

        mtx_unlock(&xnb->sc_lock);
        /* Free the network interface */
        xnb->carrier = 0;
        if (xnb->xnb_ifp != NULL) {
                ether_ifdetach(xnb->xnb_ifp);
                if_free(xnb->xnb_ifp);
                xnb->xnb_ifp = NULL;
        }
        mtx_lock(&xnb->sc_lock);

        xnb_disconnect(xnb);

        mtx_unlock(&xnb->sc_lock);
        if (xenbus_get_state(xnb->dev) < XenbusStateClosing)
                xenbus_set_state(xnb->dev, XenbusStateClosing);
        mtx_lock(&xnb->sc_lock);

        xnb->flags &= ~XNBF_IN_SHUTDOWN;


        /* Indicate to xnb_detach() that is it safe to proceed. */
        wakeup(xnb);

        return (0);
}

/**
 * Report an attach time error to the console and Xen, and cleanup
 * this instance by forcing immediate detach processing.
 *
 * \param xnb  Per-instance xnb configuration structure.
 * \param err  Errno describing the error.
 * \param fmt  Printf style format and arguments
 */
static void
xnb_attach_failed(struct xnb_softc *xnb, int err, const char *fmt, ...)
{
        va_list ap;
        va_list ap_hotplug;

        va_start(ap, fmt);
        va_copy(ap_hotplug, ap);
        xs_vprintf(XST_NIL, xenbus_get_node(xnb->dev),
                  "hotplug-error", fmt, ap_hotplug);
        va_end(ap_hotplug);
        xs_printf(XST_NIL, xenbus_get_node(xnb->dev),
                  "hotplug-status", "error");

        xenbus_dev_vfatal(xnb->dev, err, fmt, ap);
        va_end(ap);

        xs_printf(XST_NIL, xenbus_get_node(xnb->dev),
                  "online", "0");
        xnb_detach(xnb->dev);
}

/*---------------------------- NewBus Entrypoints ----------------------------*/
/**
 * Inspect a XenBus device and claim it if is of the appropriate type.
 *
 * \param dev  NewBus device object representing a candidate XenBus device.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xnb_probe(device_t dev)
{
         if (!strcmp(xenbus_get_type(dev), "vif")) {
                DPRINTF("Claiming device %d, %s\n", device_get_unit(dev),
                    devclass_get_name(device_get_devclass(dev)));
                device_set_desc(dev, "Backend Virtual Network Device");
                device_quiet(dev);
                return (0);
        }
        return (ENXIO);
}

/**
 * Setup sysctl variables to control various Network Back parameters.
 *
 * \param xnb  Xen Net Back softc.
 *
 */
static void
xnb_setup_sysctl(struct xnb_softc *xnb)
{
        struct sysctl_ctx_list *sysctl_ctx = NULL;
        struct sysctl_oid      *sysctl_tree = NULL;

        sysctl_ctx = device_get_sysctl_ctx(xnb->dev);
        if (sysctl_ctx == NULL)
                return;

        sysctl_tree = device_get_sysctl_tree(xnb->dev);
        if (sysctl_tree == NULL)
                return;

#ifdef XNB_DEBUG
        SYSCTL_ADD_PROC(sysctl_ctx,
                        SYSCTL_CHILDREN(sysctl_tree),
                        OID_AUTO,
                        "unit_test_results",
                        CTLTYPE_STRING | CTLFLAG_RD,
                        xnb,
                        0,
                        xnb_unit_test_main,
                        "A",
                        "Results of builtin unit tests");

        SYSCTL_ADD_PROC(sysctl_ctx,
                        SYSCTL_CHILDREN(sysctl_tree),
                        OID_AUTO,
                        "dump_rings",
                        CTLTYPE_STRING | CTLFLAG_RD,
                        xnb,
                        0,
                        xnb_dump_rings,
                        "A",
                        "Xennet Back Rings");
#endif /* XNB_DEBUG */
}

/**
 * Create a network device.
 * @param handle device handle
 */
int
create_netdev(device_t dev)
{
        struct ifnet *ifp;
        struct xnb_softc *xnb;
        int err = 0;
        uint32_t handle;

        xnb = device_get_softc(dev);
        mtx_init(&xnb->sc_lock, "xnb_softc", "xen netback softc lock", MTX_DEF);
        mtx_init(&xnb->tx_lock, "xnb_tx", "xen netback tx lock", MTX_DEF);
        mtx_init(&xnb->rx_lock, "xnb_rx", "xen netback rx lock", MTX_DEF);

        xnb->dev = dev;

        ifmedia_init(&xnb->sc_media, 0, xnb_ifmedia_upd, xnb_ifmedia_sts);
        ifmedia_add(&xnb->sc_media, IFM_ETHER|IFM_MANUAL, 0, NULL);
        ifmedia_set(&xnb->sc_media, IFM_ETHER|IFM_MANUAL);

        /*
         * Set the MAC address to a dummy value (00:00:00:00:00),
         * if the MAC address of the host-facing interface is set
         * to the same as the guest-facing one (the value found in
         * xenstore), the bridge would stop delivering packets to
         * us because it would see that the destination address of
         * the packet is the same as the interface, and so the bridge
         * would expect the packet has already been delivered locally
         * (and just drop it).
         */
        bzero(&xnb->mac[0], sizeof(xnb->mac));

        /* The interface will be named using the following nomenclature:
         *
         * xnb<domid>.<handle>
         *
         * Where handle is the oder of the interface referred to the guest.
         */
        err = xs_scanf(XST_NIL, xenbus_get_node(xnb->dev), "handle", NULL,
                       "%" PRIu32, &handle);
        if (err != 0)
                return (err);
        snprintf(xnb->if_name, IFNAMSIZ, "xnb%" PRIu16 ".%" PRIu32,
            xenbus_get_otherend_id(dev), handle);

        if (err == 0) {
                /* Set up ifnet structure */
                ifp = xnb->xnb_ifp = if_alloc(IFT_ETHER);
                ifp->if_softc = xnb;
                if_initname(ifp, xnb->if_name,  IF_DUNIT_NONE);
                ifp->if_flags = IFF_BROADCAST | IFF_SIMPLEX | IFF_MULTICAST;
                ifp->if_ioctl = xnb_ioctl;
                ifp->if_output = ether_output;
                ifp->if_start = xnb_start;
#ifdef notyet
                ifp->if_watchdog = xnb_watchdog;
#endif
                ifp->if_init = xnb_ifinit;
                ifp->if_mtu = ETHERMTU;
                ifp->if_snd.ifq_maxlen = NET_RX_RING_SIZE - 1;

                ifp->if_hwassist = XNB_CSUM_FEATURES;
                ifp->if_capabilities = IFCAP_HWCSUM;
                ifp->if_capenable = IFCAP_HWCSUM;

                ether_ifattach(ifp, xnb->mac);
                xnb->carrier = 0;
        }

        return err;
}

/**
 * Attach to a XenBus device that has been claimed by our probe routine.
 *
 * \param dev  NewBus device object representing this Xen Net Back instance.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xnb_attach(device_t dev)
{
        struct xnb_softc *xnb;
        int     error;
        xnb_ring_type_t i;

        error = create_netdev(dev);
        if (error != 0) {
                xenbus_dev_fatal(dev, error, "creating netdev");
                return (error);
        }

        DPRINTF("Attaching to %s\n", xenbus_get_node(dev));

        /*
         * Basic initialization.
         * After this block it is safe to call xnb_detach()
         * to clean up any allocated data for this instance.
         */
        xnb = device_get_softc(dev);
        xnb->otherend_id = xenbus_get_otherend_id(dev);
        for (i=0; i < XNB_NUM_RING_TYPES; i++) {
                xnb->ring_configs[i].ring_pages = 1;
        }

        /*
         * Setup sysctl variables.
         */
        xnb_setup_sysctl(xnb);

        /* Update hot-plug status to satisfy xend. */
        error = xs_printf(XST_NIL, xenbus_get_node(xnb->dev),
                          "hotplug-status", "connected");
        if (error != 0) {
                xnb_attach_failed(xnb, error, "writing %s/hotplug-status",
                                  xenbus_get_node(xnb->dev));
                return (error);
        }

        if ((error = xnb_publish_backend_info(xnb)) != 0) {
                /*
                 * If we can't publish our data, we cannot participate
                 * in this connection, and waiting for a front-end state
                 * change will not help the situation.
                 */
                xnb_attach_failed(xnb, error,
                    "Publishing backend status for %s",
                                  xenbus_get_node(xnb->dev));
                return error;
        }

        /* Tell the front end that we are ready to connect. */
        xenbus_set_state(dev, XenbusStateInitWait);

        return (0);
}

/**
 * Detach from a net back device instance.
 *
 * \param dev  NewBus device object representing this Xen Net Back instance.
 *
 * \return  0 for success, errno codes for failure.
 *
 * \note A net back device may be detached at any time in its life-cycle,
 *       including part way through the attach process.  For this reason,
 *       initialization order and the intialization state checks in this
 *       routine must be carefully coupled so that attach time failures
 *       are gracefully handled.
 */
static int
xnb_detach(device_t dev)
{
        struct xnb_softc *xnb;

        DPRINTF("\n");

        xnb = device_get_softc(dev);
        mtx_lock(&xnb->sc_lock);
        while (xnb_shutdown(xnb) == EAGAIN) {
                msleep(xnb, &xnb->sc_lock, /*wakeup prio unchanged*/0,
                       "xnb_shutdown", 0);
        }
        mtx_unlock(&xnb->sc_lock);
        DPRINTF("\n");

        mtx_destroy(&xnb->tx_lock);
        mtx_destroy(&xnb->rx_lock);
        mtx_destroy(&xnb->sc_lock);
        return (0);
}

/**
 * Prepare this net back device for suspension of this VM.
 *
 * \param dev  NewBus device object representing this Xen net Back instance.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xnb_suspend(device_t dev)
{
        return (0);
}

/**
 * Perform any processing required to recover from a suspended state.
 *
 * \param dev  NewBus device object representing this Xen Net Back instance.
 *
 * \return  0 for success, errno codes for failure.
 */
static int
xnb_resume(device_t dev)
{
        return (0);
}

/**
 * Handle state changes expressed via the XenStore by our front-end peer.
 *
 * \param dev             NewBus device object representing this Xen
 *                        Net Back instance.
 * \param frontend_state  The new state of the front-end.
 *
 * \return  0 for success, errno codes for failure.
 */
static void
xnb_frontend_changed(device_t dev, XenbusState frontend_state)
{
        struct xnb_softc *xnb;

        xnb = device_get_softc(dev);

        DPRINTF("frontend_state=%s, xnb_state=%s\n",
                xenbus_strstate(frontend_state),
                xenbus_strstate(xenbus_get_state(xnb->dev)));

        switch (frontend_state) {
        case XenbusStateInitialising:
                break;
        case XenbusStateInitialised:
        case XenbusStateConnected:
                xnb_connect(xnb);
                break;
        case XenbusStateClosing:
        case XenbusStateClosed:
                mtx_lock(&xnb->sc_lock);
                xnb_shutdown(xnb);
                mtx_unlock(&xnb->sc_lock);
                if (frontend_state == XenbusStateClosed)
                        xenbus_set_state(xnb->dev, XenbusStateClosed);
                break;
        default:
                xenbus_dev_fatal(xnb->dev, EINVAL, "saw state %d at frontend",
                                 frontend_state);
                break;
        }
}


/*---------------------------- Request Processing ----------------------------*/
/**
 * Interrupt handler bound to the shared ring's event channel.
 * Entry point for the xennet transmit path in netback
 * Transfers packets from the Xen ring to the host's generic networking stack
 *
 * \param arg  Callback argument registerd during event channel
 *             binding - the xnb_softc for this instance.
 */
static void
xnb_intr(void *arg)
{
        struct xnb_softc *xnb;
        struct ifnet *ifp;
        netif_tx_back_ring_t *txb;
        RING_IDX req_prod_local;

        xnb = (struct xnb_softc *)arg;
        ifp = xnb->xnb_ifp;
        txb = &xnb->ring_configs[XNB_RING_TYPE_TX].back_ring.tx_ring;

        mtx_lock(&xnb->tx_lock);
        do {
                int notify;
                req_prod_local = txb->sring->req_prod;
                xen_rmb();

                for (;;) {
                        struct mbuf *mbufc;
                        int err;

                        err = xnb_recv(txb, xnb->otherend_id, &mbufc, ifp,
                                       xnb->tx_gnttab);
                        if (err || (mbufc == NULL))
                                break;

                        /* Send the packet to the generic network stack */
                        (*xnb->xnb_ifp->if_input)(xnb->xnb_ifp, mbufc);
                }

                RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(txb, notify);
                if (notify != 0)
                        xen_intr_signal(xnb->xen_intr_handle);

                txb->sring->req_event = txb->req_cons + 1;
                xen_mb();
        } while (txb->sring->req_prod != req_prod_local) ;
        mtx_unlock(&xnb->tx_lock);

        xnb_start(ifp);
}


/**
 * Build a struct xnb_pkt based on netif_tx_request's from a netif tx ring.
 * Will read exactly 0 or 1 packets from the ring; never a partial packet.
 * \param[out]  pkt     The returned packet.  If there is an error building
 *                      the packet, pkt.list_len will be set to 0.
 * \param[in]   tx_ring Pointer to the Ring that is the input to this function
 * \param[in]   start   The ring index of the first potential request
 * \return              The number of requests consumed to build this packet
 */
static int
xnb_ring2pkt(struct xnb_pkt *pkt, const netif_tx_back_ring_t *tx_ring,
             RING_IDX start)
{
        /*
         * Outline:
         * 1) Initialize pkt
         * 2) Read the first request of the packet
         * 3) Read the extras
         * 4) Set cdr
         * 5) Loop on the remainder of the packet
         * 6) Finalize pkt (stuff like car_size and list_len)
         */
        int idx = start;
        int discard = 0;        /* whether to discard the packet */
        int more_data = 0;      /* there are more request past the last one */
        uint16_t cdr_size = 0;  /* accumulated size of requests 2 through n */

        xnb_pkt_initialize(pkt);

        /* Read the first request */
        if (RING_HAS_UNCONSUMED_REQUESTS_2(tx_ring, idx)) {
                netif_tx_request_t *tx = RING_GET_REQUEST(tx_ring, idx);
                pkt->size = tx->size;
                pkt->flags = tx->flags & ~NETTXF_more_data;
                more_data = tx->flags & NETTXF_more_data;
                pkt->list_len++;
                pkt->car = idx;
                idx++;
        }

        /* Read the extra info */
        if ((pkt->flags & NETTXF_extra_info) &&
            RING_HAS_UNCONSUMED_REQUESTS_2(tx_ring, idx)) {
                netif_extra_info_t *ext =
                    (netif_extra_info_t*) RING_GET_REQUEST(tx_ring, idx);
                pkt->extra.type = ext->type;
                switch (pkt->extra.type) {
                        case XEN_NETIF_EXTRA_TYPE_GSO:
                                pkt->extra.u.gso = ext->u.gso;
                                break;
                        default:
                                /*
                                 * The reference Linux netfront driver will
                                 * never set any other extra.type.  So we don't
                                 * know what to do with it.  Let's print an
                                 * error, then consume and discard the packet
                                 */
                                printf("xnb(%s:%d): Unknown extra info type %d."
                                       "  Discarding packet\n",
                                       __func__, __LINE__, pkt->extra.type);
                                xnb_dump_txreq(start, RING_GET_REQUEST(tx_ring,
                                    start));
                                xnb_dump_txreq(idx, RING_GET_REQUEST(tx_ring,
                                    idx));
                                discard = 1;
                                break;
                }

                pkt->extra.flags = ext->flags;
                if (ext->flags & XEN_NETIF_EXTRA_FLAG_MORE) {
                        /*
                         * The reference linux netfront driver never sets this
                         * flag (nor does any other known netfront).  So we
                         * will discard the packet.
                         */
                        printf("xnb(%s:%d): Request sets "
                            "XEN_NETIF_EXTRA_FLAG_MORE, but we can't handle "
                            "that\n", __func__, __LINE__);
                        xnb_dump_txreq(start, RING_GET_REQUEST(tx_ring, start));
                        xnb_dump_txreq(idx, RING_GET_REQUEST(tx_ring, idx));
                        discard = 1;
                }

                idx++;
        }

        /* Set cdr.  If there is not more data, cdr is invalid */
        pkt->cdr = idx;

        /* Loop on remainder of packet */
        while (more_data && RING_HAS_UNCONSUMED_REQUESTS_2(tx_ring, idx)) {
                netif_tx_request_t *tx = RING_GET_REQUEST(tx_ring, idx);
                pkt->list_len++;
                cdr_size += tx->size;
                if (tx->flags & ~NETTXF_more_data) {
                        /* There should be no other flags set at this point */
                        printf("xnb(%s:%d): Request sets unknown flags %d "
                            "after the 1st request in the packet.\n",
                            __func__, __LINE__, tx->flags);
                        xnb_dump_txreq(start, RING_GET_REQUEST(tx_ring, start));
                        xnb_dump_txreq(idx, RING_GET_REQUEST(tx_ring, idx));
                }

                more_data = tx->flags & NETTXF_more_data;
                idx++;
        }

        /* Finalize packet */
        if (more_data != 0) {
                /* The ring ran out of requests before finishing the packet */
                xnb_pkt_invalidate(pkt);
                idx = start;    /* tell caller that we consumed no requests */
        } else {
                /* Calculate car_size */
                pkt->car_size = pkt->size - cdr_size;
        }
        if (discard != 0) {
                xnb_pkt_invalidate(pkt);
        }

        return idx - start;
}


/**
 * Respond to all the requests that constituted pkt.  Builds the responses and
 * writes them to the ring, but doesn't push them to the shared ring.
 * \param[in] pkt       the packet that needs a response
 * \param[in] error     true if there was an error handling the packet, such
 *                      as in the hypervisor copy op or mbuf allocation
 * \param[out] ring     Responses go here
 */
static void
xnb_txpkt2rsp(const struct xnb_pkt *pkt, netif_tx_back_ring_t *ring,
              int error)
{
        /*
         * Outline:
         * 1) Respond to the first request
         * 2) Respond to the extra info reques
         * Loop through every remaining request in the packet, generating
         * responses that copy those requests' ids and sets the status
         * appropriately.
         */
        netif_tx_request_t *tx;
        netif_tx_response_t *rsp;
        int i;
        uint16_t status;

        status = (xnb_pkt_is_valid(pkt) == 0) || error ?
                NETIF_RSP_ERROR : NETIF_RSP_OKAY;
        KASSERT((pkt->list_len == 0) || (ring->rsp_prod_pvt == pkt->car),
            ("Cannot respond to ring requests out of order"));

        if (pkt->list_len >= 1) {
                uint16_t id;
                tx = RING_GET_REQUEST(ring, ring->rsp_prod_pvt);
                id = tx->id;
                rsp = RING_GET_RESPONSE(ring, ring->rsp_prod_pvt);
                rsp->id = id;
                rsp->status = status;
                ring->rsp_prod_pvt++;

                if (pkt->flags & NETRXF_extra_info) {
                        rsp = RING_GET_RESPONSE(ring, ring->rsp_prod_pvt);
                        rsp->status = NETIF_RSP_NULL;
                        ring->rsp_prod_pvt++;
                }
        }

        for (i=0; i < pkt->list_len - 1; i++) {
                uint16_t id;
                tx = RING_GET_REQUEST(ring, ring->rsp_prod_pvt);
                id = tx->id;
                rsp = RING_GET_RESPONSE(ring, ring->rsp_prod_pvt);
                rsp->id = id;
                rsp->status = status;
                ring->rsp_prod_pvt++;
        }
}

/**
 * Create an mbuf chain to represent a packet.  Initializes all of the headers
 * in the mbuf chain, but does not copy the data.  The returned chain must be
 * free()'d when no longer needed
 * \param[in]   pkt     A packet to model the mbuf chain after
 * \return      A newly allocated mbuf chain, possibly with clusters attached.
 *              NULL on failure
 */
static struct mbuf*
xnb_pkt2mbufc(const struct xnb_pkt *pkt, struct ifnet *ifp)
{
        /**
         * \todo consider using a memory pool for mbufs instead of
         * reallocating them for every packet
         */
        /** \todo handle extra data */
        struct mbuf *m;

        m = m_getm(NULL, pkt->size, M_NOWAIT, MT_DATA);

        if (m != NULL) {
                m->m_pkthdr.rcvif = ifp;
                if (pkt->flags & NETTXF_data_validated) {
                        /*
                         * We lie to the host OS and always tell it that the
                         * checksums are ok, because the packet is unlikely to
                         * get corrupted going across domains.
                         */
                        m->m_pkthdr.csum_flags = (
                                CSUM_IP_CHECKED |
                                CSUM_IP_VALID   |
                                CSUM_DATA_VALID |
                                CSUM_PSEUDO_HDR
                                );
                        m->m_pkthdr.csum_data = 0xffff;
                }
        }
        return m;
}

/**
 * Build a gnttab_copy table that can be used to copy data from a pkt
 * to an mbufc.  Does not actually perform the copy.  Always uses gref's on
 * the packet side.
 * \param[in]   pkt     pkt's associated requests form the src for
 *                      the copy operation
 * \param[in]   mbufc   mbufc's storage forms the dest for the copy operation
 * \param[out]  gnttab  Storage for the returned grant table
 * \param[in]   txb     Pointer to the backend ring structure
 * \param[in]   otherend_id     The domain ID of the other end of the copy
 * \return              The number of gnttab entries filled
 */
static int
xnb_txpkt2gnttab(const struct xnb_pkt *pkt, struct mbuf *mbufc,
                 gnttab_copy_table gnttab, const netif_tx_back_ring_t *txb,
                 domid_t otherend_id)
{

        struct mbuf *mbuf = mbufc;/* current mbuf within the chain */
        int gnt_idx = 0;                /* index into grant table */
        RING_IDX r_idx = pkt->car;      /* index into tx ring buffer */
        int r_ofs = 0;  /* offset of next data within tx request's data area */
        int m_ofs = 0;  /* offset of next data within mbuf's data area */
        /* size in bytes that still needs to be represented in the table */
        uint16_t size_remaining = pkt->size;

        while (size_remaining > 0) {
                const netif_tx_request_t *txq = RING_GET_REQUEST(txb, r_idx);
                const size_t mbuf_space = M_TRAILINGSPACE(mbuf) - m_ofs;
                const size_t req_size =
                        r_idx == pkt->car ? pkt->car_size : txq->size;
                const size_t pkt_space = req_size - r_ofs;
                /*
                 * space is the largest amount of data that can be copied in the
                 * grant table's next entry
                 */
                const size_t space = MIN(pkt_space, mbuf_space);

                /* TODO: handle this error condition without panicking */
                KASSERT(gnt_idx < GNTTAB_LEN, ("Grant table is too short"));

                gnttab[gnt_idx].source.u.ref = txq->gref;
                gnttab[gnt_idx].source.domid = otherend_id;
                gnttab[gnt_idx].source.offset = txq->offset + r_ofs;
                gnttab[gnt_idx].dest.u.gmfn = virt_to_mfn(
                    mtod(mbuf, vm_offset_t) + m_ofs);
                gnttab[gnt_idx].dest.offset = virt_to_offset(
                    mtod(mbuf, vm_offset_t) + m_ofs);
                gnttab[gnt_idx].dest.domid = DOMID_SELF;
                gnttab[gnt_idx].len = space;
                gnttab[gnt_idx].flags = GNTCOPY_source_gref;

                gnt_idx++;
                r_ofs += space;
                m_ofs += space;
                size_remaining -= space;
                if (req_size - r_ofs <= 0) {
                        /* Must move to the next tx request */
                        r_ofs = 0;
                        r_idx = (r_idx == pkt->car) ? pkt->cdr : r_idx + 1;
                }
                if (M_TRAILINGSPACE(mbuf) - m_ofs <= 0) {
                        /* Must move to the next mbuf */
                        m_ofs = 0;
                        mbuf = mbuf->m_next;
                }
        }

        return gnt_idx;
}

/**
 * Check the status of the grant copy operations, and update mbufs various
 * non-data fields to reflect the data present.
 * \param[in,out] mbufc mbuf chain to update.  The chain must be valid and of
 *                      the correct length, and data should already be present
 * \param[in] gnttab    A grant table for a just completed copy op
 * \param[in] n_entries The number of valid entries in the grant table
 */
static void
xnb_update_mbufc(struct mbuf *mbufc, const gnttab_copy_table gnttab,
                 int n_entries)
{
        struct mbuf *mbuf = mbufc;
        int i;
        size_t total_size = 0;

        for (i = 0; i < n_entries; i++) {
                KASSERT(gnttab[i].status == GNTST_okay,
                    ("Some gnttab_copy entry had error status %hd\n",
                    gnttab[i].status));

                mbuf->m_len += gnttab[i].len;
                total_size += gnttab[i].len;
                if (M_TRAILINGSPACE(mbuf) <= 0) {
                        mbuf = mbuf->m_next;
                }
        }
        mbufc->m_pkthdr.len = total_size;

#if defined(INET) || defined(INET6)
        xnb_add_mbuf_cksum(mbufc);
#endif
}

/**
 * Dequeue at most one packet from the shared ring
 * \param[in,out] txb   Netif tx ring.  A packet will be removed from it, and
 *                      its private indices will be updated.  But the indices
 *                      will not be pushed to the shared ring.
 * \param[in] ifnet     Interface to which the packet will be sent
 * \param[in] otherend  Domain ID of the other end of the ring
 * \param[out] mbufc    The assembled mbuf chain, ready to send to the generic
 *                      networking stack
 * \param[in,out] gnttab Pointer to enough memory for a grant table.  We make
 *                      this a function parameter so that we will take less
 *                      stack space.
 * \return              An error code
 */
static int
xnb_recv(netif_tx_back_ring_t *txb, domid_t otherend, struct mbuf **mbufc,
         struct ifnet *ifnet, gnttab_copy_table gnttab)
{
        struct xnb_pkt pkt;
        /* number of tx requests consumed to build the last packet */
        int num_consumed;
        int nr_ents;

        *mbufc = NULL;
        num_consumed = xnb_ring2pkt(&pkt, txb, txb->req_cons);
        if (num_consumed == 0)
                return 0;       /* Nothing to receive */

        /* update statistics independent of errors */
        if_inc_counter(ifnet, IFCOUNTER_IPACKETS, 1);

        /*
         * if we got here, then 1 or more requests was consumed, but the packet
         * is not necessarily valid.
         */
        if (xnb_pkt_is_valid(&pkt) == 0) {
                /* got a garbage packet, respond and drop it */
                xnb_txpkt2rsp(&pkt, txb, 1);
                txb->req_cons += num_consumed;
                DPRINTF("xnb_intr: garbage packet, num_consumed=%d\n",
                                num_consumed);
                if_inc_counter(ifnet, IFCOUNTER_IERRORS, 1);
                return EINVAL;
        }

        *mbufc = xnb_pkt2mbufc(&pkt, ifnet);

        if (*mbufc == NULL) {
                /*
                 * Couldn't allocate mbufs.  Respond and drop the packet.  Do
                 * not consume the requests
                 */
                xnb_txpkt2rsp(&pkt, txb, 1);
                DPRINTF("xnb_intr: Couldn't allocate mbufs, num_consumed=%d\n",
                    num_consumed);
                if_inc_counter(ifnet, IFCOUNTER_IQDROPS, 1);
                return ENOMEM;
        }

        nr_ents = xnb_txpkt2gnttab(&pkt, *mbufc, gnttab, txb, otherend);

        if (nr_ents > 0) {
                int __unused hv_ret = HYPERVISOR_grant_table_op(GNTTABOP_copy,
                    gnttab, nr_ents);
                KASSERT(hv_ret == 0,
                    ("HYPERVISOR_grant_table_op returned %d\n", hv_ret));
                xnb_update_mbufc(*mbufc, gnttab, nr_ents);
        }

        xnb_txpkt2rsp(&pkt, txb, 0);
        txb->req_cons += num_consumed;
        return 0;
}

/**
 * Create an xnb_pkt based on the contents of an mbuf chain.
 * \param[in] mbufc     mbuf chain to transform into a packet
 * \param[out] pkt      Storage for the newly generated xnb_pkt
 * \param[in] start     The ring index of the first available slot in the rx
 *                      ring
 * \param[in] space     The number of free slots in the rx ring
 * \retval 0            Success
 * \retval EINVAL       mbufc was corrupt or not convertible into a pkt
 * \retval EAGAIN       There was not enough space in the ring to queue the
 *                      packet
 */
static int
xnb_mbufc2pkt(const struct mbuf *mbufc, struct xnb_pkt *pkt,
              RING_IDX start, int space)
{

        int retval = 0;

        if ((mbufc == NULL) ||
             ( (mbufc->m_flags & M_PKTHDR) == 0) ||
             (mbufc->m_pkthdr.len == 0)) {
                xnb_pkt_invalidate(pkt);
                retval = EINVAL;
        } else {
                int slots_required;

                xnb_pkt_validate(pkt);
                pkt->flags = 0;
                pkt->size = mbufc->m_pkthdr.len;
                pkt->car = start;
                pkt->car_size = mbufc->m_len;

                if (mbufc->m_pkthdr.csum_flags & CSUM_TSO) {
                        pkt->flags |= NETRXF_extra_info;
                        pkt->extra.u.gso.size = mbufc->m_pkthdr.tso_segsz;
                        pkt->extra.u.gso.type = XEN_NETIF_GSO_TYPE_TCPV4;
                        pkt->extra.u.gso.pad = 0;
                        pkt->extra.u.gso.features = 0;
                        pkt->extra.type = XEN_NETIF_EXTRA_TYPE_GSO;
                        pkt->extra.flags = 0;
                        pkt->cdr = start + 2;
                } else {
                        pkt->cdr = start + 1;
                }
                if (mbufc->m_pkthdr.csum_flags & (CSUM_TSO | CSUM_DELAY_DATA)) {
                        pkt->flags |=
                            (NETRXF_csum_blank | NETRXF_data_validated);
                }

                /*
                 * Each ring response can have up to PAGE_SIZE of data.
                 * Assume that we can defragment the mbuf chain efficiently
                 * into responses so that each response but the last uses all
                 * PAGE_SIZE bytes.
                 */
                pkt->list_len = (pkt->size + PAGE_SIZE - 1) / PAGE_SIZE;

                if (pkt->list_len > 1) {
                        pkt->flags |= NETRXF_more_data;
                }

                slots_required = pkt->list_len +
                        (pkt->flags & NETRXF_extra_info ? 1 : 0);
                if (slots_required > space) {
                        xnb_pkt_invalidate(pkt);
                        retval = EAGAIN;
                }
        }

        return retval;
}

/**
 * Build a gnttab_copy table that can be used to copy data from an mbuf chain
 * to the frontend's shared buffers.  Does not actually perform the copy.
 * Always uses gref's on the other end's side.
 * \param[in]   pkt     pkt's associated responses form the dest for the copy
 *                      operatoin
 * \param[in]   mbufc   The source for the copy operation
 * \param[out]  gnttab  Storage for the returned grant table
 * \param[in]   rxb     Pointer to the backend ring structure
 * \param[in]   otherend_id     The domain ID of the other end of the copy
 * \return              The number of gnttab entries filled
 */
static int
xnb_rxpkt2gnttab(const struct xnb_pkt *pkt, const struct mbuf *mbufc,
                 gnttab_copy_table gnttab, const netif_rx_back_ring_t *rxb,
                 domid_t otherend_id)
{

        const struct mbuf *mbuf = mbufc;/* current mbuf within the chain */
        int gnt_idx = 0;                /* index into grant table */
        RING_IDX r_idx = pkt->car;      /* index into rx ring buffer */
        int r_ofs = 0;  /* offset of next data within rx request's data area */
        int m_ofs = 0;  /* offset of next data within mbuf's data area */
        /* size in bytes that still needs to be represented in the table */
        uint16_t size_remaining;

        size_remaining = (xnb_pkt_is_valid(pkt) != 0) ? pkt->size : 0;

        while (size_remaining > 0) {
                const netif_rx_request_t *rxq = RING_GET_REQUEST(rxb, r_idx);
                const size_t mbuf_space = mbuf->m_len - m_ofs;
                /* Xen shared pages have an implied size of PAGE_SIZE */
                const size_t req_size = PAGE_SIZE;
                const size_t pkt_space = req_size - r_ofs;
                /*
                 * space is the largest amount of data that can be copied in the
                 * grant table's next entry
                 */
                const size_t space = MIN(pkt_space, mbuf_space);

                /* TODO: handle this error condition without panicing */
                KASSERT(gnt_idx < GNTTAB_LEN, ("Grant table is too short"));

                gnttab[gnt_idx].dest.u.ref = rxq->gref;
                gnttab[gnt_idx].dest.domid = otherend_id;
                gnttab[gnt_idx].dest.offset = r_ofs;
                gnttab[gnt_idx].source.u.gmfn = virt_to_mfn(
                    mtod(mbuf, vm_offset_t) + m_ofs);
                gnttab[gnt_idx].source.offset = virt_to_offset(
                    mtod(mbuf, vm_offset_t) + m_ofs);
                gnttab[gnt_idx].source.domid = DOMID_SELF;
                gnttab[gnt_idx].len = space;
                gnttab[gnt_idx].flags = GNTCOPY_dest_gref;

                gnt_idx++;

                r_ofs += space;
                m_ofs += space;
                size_remaining -= space;
                if (req_size - r_ofs <= 0) {
                        /* Must move to the next rx request */
                        r_ofs = 0;
                        r_idx = (r_idx == pkt->car) ? pkt->cdr : r_idx + 1;
                }
                if (mbuf->m_len - m_ofs <= 0) {
                        /* Must move to the next mbuf */
                        m_ofs = 0;
                        mbuf = mbuf->m_next;
                }
        }

        return gnt_idx;
}

/**
 * Generates responses for all the requests that constituted pkt.  Builds
 * responses and writes them to the ring, but doesn't push the shared ring
 * indices.
 * \param[in] pkt       the packet that needs a response
 * \param[in] gnttab    The grant copy table corresponding to this packet.
 *                      Used to determine how many rsp->netif_rx_response_t's to
 *                      generate.
 * \param[in] n_entries Number of relevant entries in the grant table
 * \param[out] ring     Responses go here
 * \return              The number of RX requests that were consumed to generate
 *                      the responses
 */
static int
xnb_rxpkt2rsp(const struct xnb_pkt *pkt, const gnttab_copy_table gnttab,
              int n_entries, netif_rx_back_ring_t *ring)
{
        /*
         * This code makes the following assumptions:
         *      * All entries in gnttab set GNTCOPY_dest_gref
         *      * The entries in gnttab are grouped by their grefs: any two
         *         entries with the same gref must be adjacent
         */
        int error = 0;
        int gnt_idx, i;
        int n_responses = 0;
        grant_ref_t last_gref = GRANT_REF_INVALID;
        RING_IDX r_idx;

        KASSERT(gnttab != NULL, ("Received a null granttable copy"));

        /*
         * In the event of an error, we only need to send one response to the
         * netfront.  In that case, we musn't write any data to the responses
         * after the one we send.  So we must loop all the way through gnttab
         * looking for errors before we generate any responses
         *
         * Since we're looping through the grant table anyway, we'll count the
         * number of different gref's in it, which will tell us how many
         * responses to generate
         */
        for (gnt_idx = 0; gnt_idx < n_entries; gnt_idx++) {
                int16_t status = gnttab[gnt_idx].status;
                if (status != GNTST_okay) {
                        DPRINTF(
                            "Got error %d for hypervisor gnttab_copy status\n",
                            status);
                        error = 1;
                        break;
                }
                if (gnttab[gnt_idx].dest.u.ref != last_gref) {
                        n_responses++;
                        last_gref = gnttab[gnt_idx].dest.u.ref;
                }
        }

        if (error != 0) {
                uint16_t id;
                netif_rx_response_t *rsp;
                
                id = RING_GET_REQUEST(ring, ring->rsp_prod_pvt)->id;
                rsp = RING_GET_RESPONSE(ring, ring->rsp_prod_pvt);
                rsp->id = id;
                rsp->status = NETIF_RSP_ERROR;
                n_responses = 1;
        } else {
                gnt_idx = 0;
                const int has_extra = pkt->flags & NETRXF_extra_info;
                if (has_extra != 0)
                        n_responses++;

                for (i = 0; i < n_responses; i++) {
                        netif_rx_request_t rxq;
                        netif_rx_response_t *rsp;

                        r_idx = ring->rsp_prod_pvt + i;
                        /*
                         * We copy the structure of rxq instead of making a
                         * pointer because it shares the same memory as rsp.
                         */
                        rxq = *(RING_GET_REQUEST(ring, r_idx));
                        rsp = RING_GET_RESPONSE(ring, r_idx);
                        if (has_extra && (i == 1)) {
                                netif_extra_info_t *ext =
                                        (netif_extra_info_t*)rsp;
                                ext->type = XEN_NETIF_EXTRA_TYPE_GSO;
                                ext->flags = 0;
                                ext->u.gso.size = pkt->extra.u.gso.size;
                                ext->u.gso.type = XEN_NETIF_GSO_TYPE_TCPV4;
                                ext->u.gso.pad = 0;
                                ext->u.gso.features = 0;
                        } else {
                                rsp->id = rxq.id;
                                rsp->status = GNTST_okay;
                                rsp->offset = 0;
                                rsp->flags = 0;
                                if (i < pkt->list_len - 1)
                                        rsp->flags |= NETRXF_more_data;
                                if ((i == 0) && has_extra)
                                        rsp->flags |= NETRXF_extra_info;
                                if ((i == 0) &&
                                        (pkt->flags & NETRXF_data_validated)) {
                                        rsp->flags |= NETRXF_data_validated;
                                        rsp->flags |= NETRXF_csum_blank;
                                }
                                rsp->status = 0;
                                for (; gnttab[gnt_idx].dest.u.ref == rxq.gref;
                                    gnt_idx++) {
                                        rsp->status += gnttab[gnt_idx].len;
                                }
                        }
                }
        }

        ring->req_cons += n_responses;
        ring->rsp_prod_pvt += n_responses;
        return n_responses;
}

#if defined(INET) || defined(INET6)
/**
 * Add IP, TCP, and/or UDP checksums to every mbuf in a chain.  The first mbuf
 * in the chain must start with a struct ether_header.
 *
 * XXX This function will perform incorrectly on UDP packets that are split up
 * into multiple ethernet frames.
 */
static void
xnb_add_mbuf_cksum(struct mbuf *mbufc)
{
        struct ether_header *eh;
        struct ip *iph;
        uint16_t ether_type;

        eh = mtod(mbufc, struct ether_header*);
        ether_type = ntohs(eh->ether_type);
        if (ether_type != ETHERTYPE_IP) {
                /* Nothing to calculate */
                return;
        }

        iph = (struct ip*)(eh + 1);
        if (mbufc->m_pkthdr.csum_flags & CSUM_IP_VALID) {
                iph->ip_sum = 0;
                iph->ip_sum = in_cksum_hdr(iph);
        }

        switch (iph->ip_p) {
        case IPPROTO_TCP:
                if (mbufc->m_pkthdr.csum_flags & CSUM_IP_VALID) {
                        size_t tcplen = ntohs(iph->ip_len) - sizeof(struct ip);
                        struct tcphdr *th = (struct tcphdr*)(iph + 1);
                        th->th_sum = in_pseudo(iph->ip_src.s_addr,
                            iph->ip_dst.s_addr, htons(IPPROTO_TCP + tcplen));
                        th->th_sum = in_cksum_skip(mbufc,
                            sizeof(struct ether_header) + ntohs(iph->ip_len),
                            sizeof(struct ether_header) + (iph->ip_hl << 2));
                }
                break;
        case IPPROTO_UDP:
                if (mbufc->m_pkthdr.csum_flags & CSUM_IP_VALID) {
                        size_t udplen = ntohs(iph->ip_len) - sizeof(struct ip);
                        struct udphdr *uh = (struct udphdr*)(iph + 1);
                        uh->uh_sum = in_pseudo(iph->ip_src.s_addr,
                            iph->ip_dst.s_addr, htons(IPPROTO_UDP + udplen));
                        uh->uh_sum = in_cksum_skip(mbufc,
                            sizeof(struct ether_header) + ntohs(iph->ip_len),
                            sizeof(struct ether_header) + (iph->ip_hl << 2));
                }
                break;
        default:
                break;
        }
}
#endif /* INET || INET6 */

static void
xnb_stop(struct xnb_softc *xnb)
{
        struct ifnet *ifp;

        mtx_assert(&xnb->sc_lock, MA_OWNED);
        ifp = xnb->xnb_ifp;
        ifp->if_drv_flags &= ~(IFF_DRV_RUNNING | IFF_DRV_OACTIVE);
        if_link_state_change(ifp, LINK_STATE_DOWN);
}

static int
xnb_ioctl(struct ifnet *ifp, u_long cmd, caddr_t data)
{
        struct xnb_softc *xnb = ifp->if_softc;
        struct ifreq *ifr = (struct ifreq*) data;
#ifdef INET
        struct ifaddr *ifa = (struct ifaddr*)data;
#endif
        int error = 0;

        switch (cmd) {
                case SIOCSIFFLAGS:
                        mtx_lock(&xnb->sc_lock);
                        if (ifp->if_flags & IFF_UP) {
                                xnb_ifinit_locked(xnb);
                        } else {
                                if (ifp->if_drv_flags & IFF_DRV_RUNNING) {
                                        xnb_stop(xnb);
                                }
                        }
                        /*
                         * Note: netfront sets a variable named xn_if_flags
                         * here, but that variable is never read
                         */
                        mtx_unlock(&xnb->sc_lock);
                        break;
                case SIOCSIFADDR:
#ifdef INET
                        mtx_lock(&xnb->sc_lock);
                        if (ifa->ifa_addr->sa_family == AF_INET) {
                                ifp->if_flags |= IFF_UP;
                                if (!(ifp->if_drv_flags & IFF_DRV_RUNNING)) {
                                        ifp->if_drv_flags &= ~(IFF_DRV_RUNNING |
                                                        IFF_DRV_OACTIVE);
                                        if_link_state_change(ifp,
                                                        LINK_STATE_DOWN);
                                        ifp->if_drv_flags |= IFF_DRV_RUNNING;
                                        ifp->if_drv_flags &= ~IFF_DRV_OACTIVE;
                                        if_link_state_change(ifp,
                                            LINK_STATE_UP);
                                }
                                arp_ifinit(ifp, ifa);
                                mtx_unlock(&xnb->sc_lock);
                        } else {
                                mtx_unlock(&xnb->sc_lock);
#endif
                                error = ether_ioctl(ifp, cmd, data);
#ifdef INET
                        }
#endif
                        break;
                case SIOCSIFCAP:
                        mtx_lock(&xnb->sc_lock);
                        if (ifr->ifr_reqcap & IFCAP_TXCSUM) {
                                ifp->if_capenable |= IFCAP_TXCSUM;
                                ifp->if_hwassist |= XNB_CSUM_FEATURES;
                        } else {
                                ifp->if_capenable &= ~(IFCAP_TXCSUM);
                                ifp->if_hwassist &= ~(XNB_CSUM_FEATURES);
                        }
                        if ((ifr->ifr_reqcap & IFCAP_RXCSUM)) {
                                ifp->if_capenable |= IFCAP_RXCSUM;
                        } else {
                                ifp->if_capenable &= ~(IFCAP_RXCSUM);
                        }
                        /*
                         * TODO enable TSO4 and LRO once we no longer need
                         * to calculate checksums in software
                         */
#if 0
                        if (ifr->if_reqcap |= IFCAP_TSO4) {
                                if (IFCAP_TXCSUM & ifp->if_capenable) {
                                        printf("xnb: Xen netif requires that "
                                                "TXCSUM be enabled in order "
                                                "to use TSO4\n");
                                        error = EINVAL;
                                } else {
                                        ifp->if_capenable |= IFCAP_TSO4;
                                        ifp->if_hwassist |= CSUM_TSO;
                                }
                        } else {
                                ifp->if_capenable &= ~(IFCAP_TSO4);
                                ifp->if_hwassist &= ~(CSUM_TSO);
                        }
                        if (ifr->ifreqcap |= IFCAP_LRO) {
                                ifp->if_capenable |= IFCAP_LRO;
                        } else {
                                ifp->if_capenable &= ~(IFCAP_LRO);
                        }
#endif
                        mtx_unlock(&xnb->sc_lock);
                        break;
                case SIOCSIFMTU:
                        ifp->if_mtu = ifr->ifr_mtu;
                        ifp->if_drv_flags &= ~IFF_DRV_RUNNING;
                        xnb_ifinit(xnb);
                        break;
                case SIOCADDMULTI:
                case SIOCDELMULTI:
                case SIOCSIFMEDIA:
                case SIOCGIFMEDIA:
                        error = ifmedia_ioctl(ifp, ifr, &xnb->sc_media, cmd);
                        break;
                default:
                        error = ether_ioctl(ifp, cmd, data);
                        break;
        }
        return (error);
}

static void
xnb_start_locked(struct ifnet *ifp)
{
        netif_rx_back_ring_t *rxb;
        struct xnb_softc *xnb;
        struct mbuf *mbufc;
        RING_IDX req_prod_local;

        xnb = ifp->if_softc;
        rxb = &xnb->ring_configs[XNB_RING_TYPE_RX].back_ring.rx_ring;

        if (!xnb->carrier)
                return;

        do {
                int out_of_space = 0;
                int notify;
                req_prod_local = rxb->sring->req_prod;
                xen_rmb();
                for (;;) {
                        int error;

                        IF_DEQUEUE(&ifp->if_snd, mbufc);
                        if (mbufc == NULL)
                                break;
                        error = xnb_send(rxb, xnb->otherend_id, mbufc,
                                         xnb->rx_gnttab);
                        switch (error) {
                                case EAGAIN:
                                        /*
                                         * Insufficient space in the ring.
                                         * Requeue pkt and send when space is
                                         * available.
                                         */
                                        IF_PREPEND(&ifp->if_snd, mbufc);
                                        /*
                                         * Perhaps the frontend missed an IRQ
                                         * and went to sleep.  Notify it to wake
                                         * it up.
                                         */
                                        out_of_space = 1;
                                        break;

                                case EINVAL:
                                        /* OS gave a corrupt packet.  Drop it.*/
                                        if_inc_counter(ifp, IFCOUNTER_OERRORS, 1);
                                        /* FALLTHROUGH */
                                default:
                                        /* Send succeeded, or packet had error.
                                         * Free the packet */
                                        if_inc_counter(ifp, IFCOUNTER_OPACKETS, 1);
                                        if (mbufc)
                                                m_freem(mbufc);
                                        break;
                        }
                        if (out_of_space != 0)
                                break;
                }

                RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(rxb, notify);
                if ((notify != 0) || (out_of_space != 0))
                        xen_intr_signal(xnb->xen_intr_handle);
                rxb->sring->req_event = req_prod_local + 1;
                xen_mb();
        } while (rxb->sring->req_prod != req_prod_local) ;
}

/**
 * Sends one packet to the ring.  Blocks until the packet is on the ring
 * \param[in]   mbufc   Contains one packet to send.  Caller must free
 * \param[in,out] rxb   The packet will be pushed onto this ring, but the
 *                      otherend will not be notified.
 * \param[in]   otherend The domain ID of the other end of the connection
 * \retval      EAGAIN  The ring did not have enough space for the packet.
 *                      The ring has not been modified
 * \param[in,out] gnttab Pointer to enough memory for a grant table.  We make
 *                      this a function parameter so that we will take less
 *                      stack space.
 * \retval EINVAL       mbufc was corrupt or not convertible into a pkt
 */
static int
xnb_send(netif_rx_back_ring_t *ring, domid_t otherend, const struct mbuf *mbufc,
         gnttab_copy_table gnttab)
{
        struct xnb_pkt pkt;
        int error, n_entries, n_reqs;
        RING_IDX space;

        space = ring->sring->req_prod - ring->req_cons;
        error = xnb_mbufc2pkt(mbufc, &pkt, ring->rsp_prod_pvt, space);
        if (error != 0)
                return error;
        n_entries = xnb_rxpkt2gnttab(&pkt, mbufc, gnttab, ring, otherend);
        if (n_entries != 0) {
                int __unused hv_ret = HYPERVISOR_grant_table_op(GNTTABOP_copy,
                    gnttab, n_entries);
                KASSERT(hv_ret == 0, ("HYPERVISOR_grant_table_op returned %d\n",
                    hv_ret));
        }

        n_reqs = xnb_rxpkt2rsp(&pkt, gnttab, n_entries, ring);

        return 0;
}

static void
xnb_start(struct ifnet *ifp)
{
        struct xnb_softc *xnb;

        xnb = ifp->if_softc;
        mtx_lock(&xnb->rx_lock);
        xnb_start_locked(ifp);
        mtx_unlock(&xnb->rx_lock);
}

/* equivalent of network_open() in Linux */
static void
xnb_ifinit_locked(struct xnb_softc *xnb)
{
        struct ifnet *ifp;

        ifp = xnb->xnb_ifp;

        mtx_assert(&xnb->sc_lock, MA_OWNED);

        if (ifp->if_drv_flags & IFF_DRV_RUNNING)
                return;

        xnb_stop(xnb);

        ifp->if_drv_flags |= IFF_DRV_RUNNING;
        ifp->if_drv_flags &= ~IFF_DRV_OACTIVE;
        if_link_state_change(ifp, LINK_STATE_UP);
}


static void
xnb_ifinit(void *xsc)
{
        struct xnb_softc *xnb = xsc;

        mtx_lock(&xnb->sc_lock);
        xnb_ifinit_locked(xnb);
        mtx_unlock(&xnb->sc_lock);
}

/**
 * Callback used by the generic networking code to tell us when our carrier
 * state has changed.  Since we don't have a physical carrier, we don't care
 */
static int
xnb_ifmedia_upd(struct ifnet *ifp)
{
        return (0);
}

/**
 * Callback used by the generic networking code to ask us what our carrier
 * state is.  Since we don't have a physical carrier, this is very simple
 */
static void
xnb_ifmedia_sts(struct ifnet *ifp, struct ifmediareq *ifmr)
{
        ifmr->ifm_status = IFM_AVALID|IFM_ACTIVE;
        ifmr->ifm_active = IFM_ETHER|IFM_MANUAL;
}


/*---------------------------- NewBus Registration ---------------------------*/
static device_method_t xnb_methods[] = {
        /* Device interface */
        DEVMETHOD(device_probe,         xnb_probe),
        DEVMETHOD(device_attach,        xnb_attach),
        DEVMETHOD(device_detach,        xnb_detach),
        DEVMETHOD(device_shutdown,      bus_generic_shutdown),
        DEVMETHOD(device_suspend,       xnb_suspend),
        DEVMETHOD(device_resume,        xnb_resume),

        /* Xenbus interface */
        DEVMETHOD(xenbus_otherend_changed, xnb_frontend_changed),

        { 0, 0 }
};

static driver_t xnb_driver = {
        "xnb",
        xnb_methods,
        sizeof(struct xnb_softc),
};
devclass_t xnb_devclass;

DRIVER_MODULE(xnb, xenbusb_back, xnb_driver, xnb_devclass, 0, 0);


/*-------------------------- Unit Tests -------------------------------------*/
#ifdef XNB_DEBUG
#include "netback_unit_tests.c"
#endif